In general, one of the brake shoes made by rubber of a bicycle may be abraded more seriously after being used for a long time, or two brake shoes do not balance after assembling.
Because of unbalance, the efficiency of braking may be reduced, and it may cause noise, even vibration while braking.
It is difficult or complex to fine tune the distance between two brake shoes. Therefore, the users usually ignore it to adjust.
However, the safety for a rider is not enough. If the brake shoes are not balanced, the friction is not equal and it is easy to reduce the braking efficiency, and then the safety of riders may be influenced.